is cellulose abstractly hard to digest, or is it just that the animal lineages we got issued are all bad at it, and so have to implement complex Rube Goldberg solutions. there are bacteria and protists (such as those working as symbiotes in termites, cows, and other macroscopic cellulose-eating animals) that seem to be okay at it - maybe the reengineered shambling flannel could learn to use some of their enzymes.